The Hierarchy are a Delta Quadrant species who were encountered by the USS Voyager in 2376. They appear to be raiders, attacking those ships which pass through their space in order to steal resources. They hide vessels in nebulas, from which they conduct long range scans to find potential targets. Once a target comes into range the Hierarchy conduct detailed scans, analysing its defensive capability and usefulness.
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Physically the Hierarchy are Humanoid, heavily built with brown hairless skin. They tend to limit their individuality somewhat, passing all information to a hierarchical command structure which makes the decisions. This hierarchy system is so ingrained that they have even adopted the term as their species name, although at least some members of their society resent the degree of control which the Hierarchy has over their lives.
When Voyager came within range of a Hierarchy Attack class ship they initially assessed the Starfleet vessel as being a difficult target. However, one of their operatives was able to gain access to the EHM program to gain an idea of the ships capabilities. Despite some confusion caused by an experimental algorithm the EMH was trying at the time, the Hierarchy decided to attack. One of them, having come to feel a certain friendship for the EMH during his observations, informed Voyager's crew of the impending attack. They were able to bluff the Hierarchy ships into retreating and continue on their way. 
Voyager had two further encounters with the Hierarchy, both times with individuals separated from the command structure in some way.  The first came when Voyager was trapped within a subspace anomaly, and forged alliances with several other ships in order to escape; a Hierarchy ship was part of this alliance.  Later Voyager encountered two rogue Hierarchy members, who kidnapped Captain Janeway to blackmail the EMH into stealing Voyager's warp core for them.